The present invention relates to expansion devices employed in refrigeration systems. More particularly, the present invention relates to a fixed orifice expansion device used in refrigeration chillers.
Refrigeration chillers are constituted of four basic components: a compressor, a condenser, an expansion device and an evaporator. These components are connected for flow to form a refrigeration circuit the most typical purpose of which is to chill a liquid such as water. The chilled liquid is most often used to comfort condition buildings or in industrial process applications.
In larger chillers employing multi-stage compressor, a so-called economizer component and a second expansion device is often employed for the purpose of enhancing overall chiller efficiency. A multi-stage compressor is one which compresses refrigerant gas more than once and in a stepwise fashion.
In that regard, in a chiller employing a compressor of the centrifugal type, one, two or more stages of compression may exist within the compressor. Compression is accomplished in each such stage by the high speed rotation of an impeller in a volute. Refrigerant gas is directed axially into the inlet portion of the impeller and is accelerated therethrough by the rotation of the impeller. By virtue of the acceleration and the decreasing volume through which the gas passes as it flows through the impeller, compression is achieved. Compressed gas exits the impeller, is collected in the volute and is then directed to the inlet of the impeller associated with the next stage of compression.
After the final stage of compression, relatively hot compressed refrigerant gas is discharged from the compressor into the chiller's condenser where it condenses as it undergoes heat exchange with a cooling fluid, most typically water sourced from a cooling tower, municipal water system or the like. The relatively warm and still relatively high pressure liquid refrigerant is directed out of the condenser, to and through an expansion device.
The simplest and most inexpensive form of expansion device used in refrigeration chillers is a fixed orifice which is sized in accordance with the particular application in which the chiller is employed. As a result of the pressure drop that occurs as the refrigerant flows through the expansion device, a portion of the liquid refrigerant flashes to gaseous form. As a result of that change of state, the refrigerant experiences a further cooling effect.
Because the portion of the refrigerant that flashes to gas in passing through the expansion device will still be at relatively high pressure, it can be collected in an economizer component and directed to a lower pressure, inter-stage location within the multi-stage compressor of a refrigeration chiller. The removal of this flash gas from the stream of refrigerant flowing to the evaporator increases the heat absorption capability of the refrigerant that makes its way into the evaporator and reduces the energy required to compress the gas that flows to the compressor from the evaporator. Chiller efficiency is enhanced thereby.
The portion of the refrigerant that remains in liquid form in the economizer flows from the economizer and to and through a second expansion device prior to entering the system evaporator. After entering the evaporator, the refrigerant undergoes heat exchange with and cools a liquid flowing therethrough. That liquid is ultimately used to cool the heat load it is the purpose of the chiller to cool. As a result of the heat exchange within the evaporator, the refrigerant vaporizes and is drawn, at relatively low pressure and temperature back to the first stage of the system compressor.
Heretofore, the use of a fixed orifice expansion devices to control refrigerant flow from the system condenser, while relatively simple and inexpensive, has generally resulted in less than optimal chiller performance under certain operating conditions that can exist at the edges of a chiller's operating envelope. In that regard, when so-called high head, low flow conditions exist with respect to the chiller's condenser, as in the region indicated in zone A in FIG. 1, there will typically be insufficient liquid refrigerant in the condenser to ensure that a continuous liquid seal exists at the location of the orifice located downstream of the condenser.
If a liquid seal is not maintained at the orifice location, hot refrigerant gas can flow directly out of the condenser and through the orifice without having undergone heat exchange with the cooling medium flowing through the condenser. The net effect of this circumstance, referred to as hot gas bypass, is to waste power for failure of the heat in the compressed refrigerant gas that is mechanically produced by the compressor to be rejected to the cooling medium flowing through the condenser.
On the other hand, under so-called high flow, low head conditions, as in the region indicated by zone B in FIG. 1, insufficient pressure will typically exist within the condenser to drive liquid refrigerant out of the condenser through the orifice in the quantity that is available and which the system is capable of using for cooling the downstream heat load. Under that circumstance, the chiller, in essence, uses more power to achieve less cooling which is, again, energy wasteful.
Overall, at high head, low flow conditions, the orifice downstream of the system condenser would advantageously be smaller, to ensure that hot gas bypass does not occur while at high flow, low head conditions, the orifice would advantageously be larger to ensure that the quantity of liquid refrigerant produced in the system condenser is capable of being delivered through the downstream expansion device to effect cooling. Under "normal" chiller operating conditions orifice size will be intermediate such small and large sizes so as to provide for refrigerant flow that is optimized for the particular chiller application and the "normal" conditions under which the chiller operates in the circumstances of its application.
While chillers can employ electronic expansion valves that vary orifice sizing to achieve such results, the use of such devices often cannot be justified due to the expense and system complication their use brings, in terms of additional sensors and controls associated with their use and the fact that high flow/low head and/or high head/low flow conditions may exist only rarely in the circumstances of the particular application in which a chiller is employed. The need therefore continues to exist for a fixed orifice expansion device for a refrigeration chiller that is relatively inexpensive, is simple to manufacture and control and is capable of appropriately governing refrigerant flow out of the system condenser even when the chiller is operating under conditions at the edges of its operating envelope.